1. Field of the Invention
The present invention relates to high-frequency amplifier circuits for UHF television tuners.
2. Description of the Related Art
Hereinafter, a known art is described with reference to FIGS. 3, 6, and 7. FIG. 6 shows a known high-frequency amplifier circuit for a UHF television tuner (hereinafter referred to as a high-frequency amplifier circuit). UHF band television signals are input via an input tuning circuit 21 to a first gate G1 of a dual gate FET 22. Connected to the first gate G1 is a peaking coil 23 for compensating gain reduction of a low UHF band, and a bias voltage is applied from a power supply B to the first gate G1 via the peaking coil 23. An AGC voltage A for varying the gain of the FET 22 is applied to a second gate G2 via a feed resistor 24. The second gate G2 is grounded in a high-frequency state via a grounding capacitor 25, which has a capacitance of several nF. A source S is also grounded in a high-frequency state.
The amplified UHF band television signals are output from a drain D and input to an interstage tuning circuit 26. Then, UHF band television signals to be received are selected there and the selected signals are input to a mixer (not shown) in the next stage.
FIG. 7 shows a circuit diagram in which the FET 22 is equivalently represented with two single-gate FETs each having a grounded source or a grounded gate. Herein, it is difficult to securely ground the second gate G2 in a high-frequency state due to the residual inductance of the grounding capacitor 25 and the inductance of a wiring conductor for connecting the grounding capacitor 25. Actually, the second gate G2 is grounded via an inductance component L having a low inductance. The impedance of the inductance component L is not negligible in the UHF band, because it generates distortion in an operation of the FET 22 and deteriorates a cross modulation characteristic.
The cross modulation characteristic is defined by a level of a disturbing signal which causes 1% of cross modulation. Accordingly, the higher the level is, the better cross modulation characteristic is. Since the cross modulation occurs in the high-frequency amplifier circuit, a variation in the level to attenuation in the gain of the high-frequency amplifier circuit is measured in general. In FIG. 3, X1 represents a cross modulation characteristic in the known art. The cross modulation characteristic is deteriorated when attenuation in the gain is 25 dB or more, and the level of the disturbing signal is lowered to 96 dBxcexc at 45 dB, thus easily causing cross modulation. Although the reason for this is not clear, a possible reason is a phase when the FET at the grounded gate is fed back negatively by the inductance component L.
Accordingly, it is an object of the present invention to provide a high-frequency amplifier circuit for a UHF television tuner, in which a dual gate FET of the circuit reduces distortion generated when a UHF band television signal is amplified.
To this end, according to the present invention, a high-frequency amplifier circuit for a UHF television tuner comprises an FET having a first gate for inputting a UHF band television signal and a second gate for applying an AGC voltage which varies the gain. The second gate is grounded via a series circuit including a resistor and a DC-cutting capacitor. The impedance of the DC-cutting capacitor is sufficiently lower than the resistance of the resistor in the UHF band.
With these features, the FET is fed back by the resistance, and distortion is improved.
Preferably, the resistor is connected to the second gate, the DC-cutting capacitor is grounded, and the AGC voltage is applied to the node between the resistor and the DC-cutting capacitor.
Accordingly, the voltage source impedance of the AGC voltage is not coupled to the second gate and the second gate is grounded securely via only the resistor.
Further, the resistance of the resistor may be in the range of 25 to 100xcexa9. In such a case, the gain and noise figure are slightly degraded while the cross modulation and distortion of PCS beat are remarkably improved.